This invention relates to drill bits, and more particularly to drill bits with diamond cutters used in the rotary drilling of bore holes in earth formations.
Typically, earth boring drill bits include an integral bit body which may be of steel or may be fabricated of a hard matrix material such as tungsten carbide. A plurality of diamond cutter devices are mounted along the exterior face of the bit body. Each diamond cutter typically has a stud portion which is mounted in a recess in the exterior face of the bit body. Depending upon the design of the bit body and the type of diamonds used (i.e., either natural or synthetic), the cutters are either positioned in a mold prior to formation of the bit body or are secured to the bit body after fabrication.
The cutting elements are positioned along the leading edges of the bit body so that as the bit body is rotated in its intended direction of use, the cutting elements engage and drill the earth formation. In use, tremendous forces are exerted on the cutting elements, particularly in the forward to rear direction. Additionally, the bit and cutting elements are subjected to substantial abrasive forces. In some instances, impact, lateral and/or abrasive forces have caused drill bit failure and cutter loss.
While steel body bits have toughness and ductility properties which render them resistant to cracking and failure due to impact forces generated during drilling, steel is subject to rapid erosion due to abrasive forces, such as high velocity drilling fluids, during drilling. Generally, such steel body bits are coated with a more erosion resistant material such as tungsten carbide to improve their erosion resistance. However, tungsten carbide and other erosion resistant materials are brittle. During use, the relatively thin coatings may crack and peel, revealing the softer steel body which is then rapidly eroded. This leads to cutter loss, as the area around the cutter is eroded away, and eventual failure of the bit.
Tungsten carbide or other hard metal matrix bits have the advantage of high erosion resistance. The matrix bit is generally formed by packing a graphite mold with tungsten carbide powder and then infiltrating the powder with a molten copper alloy binder. A steel blank is present in the mold and becomes secured to the matrix. The end of the blank can then be welded or otherwise secured to an upper threaded body portion of the bit.
Such tungsten carbide or other hard metal matrix bits, however, are brittle and can crack upon being subjected to impact forces encountered during drilling. Additionally, thermal stresses from the heat generated during fabrication of the bit or during drilling may cause cracks to form. Typically, such cracks occur where the cutter elements have been secured to the matrix body. If the cutter elements are sheared from the drill bit body, the expensive diamonds on the cutter elements are lost, and the bit may cease to drill. Additionally, tungsten carbide is very expensive in comparison with steel as a material of fabrication.
Accordingly, there is a need in the art for a drill bit which has the toughness, ductility, and impact strength of steel and the hardness and erosion resistance of tungsten carbide or other hard metal on the exterior surface, but without the problems of prior art steel body and hard metal matrix body bits. There is also a need in the art for an erosion resistant bit with a lower total cost.